Gas turbines, in the case of which a power-generating generator is driven by a gas turbine and the generated electric power is fed to a grid with a predetermined grid frequency (for example 50 or 60 Hz), can have a fixed connection between the (mechanical) speed of the turbine and the grid frequency. The power output of the generator in this case is connected in a frequency-locked manner to the grid via a grid connection, while it is driven in a speed-coupled manner by the turbine either directly (1-shaft arrangement) or via a mechanical transmission.
If large consumers are connected to the grid or power plants fall off the grid, fluctuations of the grid frequency can occur, which the gas turbine has to directly follow as a result of the frequency-locked coupling. This can lead to critical operating conditions in the compressor of a gas turbine.
FIG. 1 shows in a greatly simplified view a power plant 10 which can be operated in accordance with exemplary methods as disclosed herein, and which generates power by means of a gas turbine 12 with coupled first generator 18 and a steam turbine 24 with coupled second generator 18, to feed the power to a grid. The gas turbine 12 and the generator 18 are connected by means of a common shaft 19 and form a shaft train 11. The gas turbine in a simple case comprises a compressor 13 with associated variable compressor guide vanes and controller via an air intake 16, inducts and compresses combustion air. The compressor 13 can be assembled from a plurality of series-connected compressor sections which operate on increasing pressure level, and possibly enable intercooling of the compressed air. The combustion air which is compressed in the compressor 13 reaches the combustion chamber 15 into which liquid fuel (for example oil or other suitable fuel) or gaseous fuel (for example, natural gas or other gaseous fuel) is injected via a fuel feed line 17 and, consuming combustion air, is combusted.
The hot gases which issue from the combustion chamber 15 are expanded in a subsequent turbine 14, performing work, and so drive the compressor 13 and the coupled first generator 18. The still relatively hot exhaust gas when discharging from the turbine is transmitted through a subsequent heat recovery steam generator 23 in order to produce steam in a separate water-steam cycle 25 for the operation of a steam turbine 24. Condenser, feed-water pump and further systems of the water-steam cycle 25 are not shown for simplification of the figure. Such a combination of gas turbine and steam power plant is referred to as a combined cycle power plant. The steam turbine 24 in this case can be coupled with the first generator 18 on the side opposite the turbine 14; gas turbine 12, first generator 18 and steam turbine 24 then form a so-called “single-shaft power train”. The steam turbine 24, however, as shown in FIG. 1, can also drive a separate second generator 18 on a separate shaft train. Different combinations are known for multi-shaft arrangements.
In the case of the 1-shaft gas turbine of FIG. 1, the speed of the gas turbine 12 can be in a fixed ratio to the frequency, which is generated in the generator 18, of the alternating voltage which is equal to the grid frequency of the grid 21. As a result, the mechanical speed with which the compressor of the gas turbine is operated is also determined.
The frequency range, and therefore also a minimum mechanical speed, in which the gas turbine should be able to operate, as a rule is stipulated by the grid operator. Furthermore, an ambient operating range is defined, depending upon the local circumstances. In order to be able to safely operate within the entire possible frequency range and ambient operating range, the compressor of the gas turbine is designed and operated with a margin to the so-called surge limit.
The surge limit is the state at which the pressure ratio which has to be built up by the compressor for the current operating condition becomes too large, and at which a flow separation, backflow and pressure surges in the compressor can occur.
A stability criterion for the safe operation of a compressor is the so-called “aerodynamic speed”. For the respective positioning of variable compressor guide vanes, the maximum pressure ratio which the compressor can build up is created in dependence upon the aerodynamic speed. In order to ensure a safe operation of the gas turbine, this is operated with a margin to the surge limit, which is also referred to as surge limit margin.
If the margin to the surge limit is no longer sufficient for a safe operation of the gas turbine, the compressor should be unloaded. This can be achieved by means of a reduction of the compressor pressure ratio or by closing the variable compressor inlet guide vanes.
These measures lead to a reduction of the power output and in an optimized power plant operation are to be avoided as far as possible. For example, in the case of an underfrequency event on days with high ambient temperature in which the gas turbine power output decreases proportionally to the grid frequency without further countermeasures, a further power output reduction would additionally destabilize the grid.
Furthermore, the gas turbine can be taken off the grid by protective unloading, such as by means for load shedding, and further operated independently of the grid in a speed-governed manner. Since it then delivers no more power to the grid, however, this is an unfavorable solution for grid stability.
The operating of gas turbines with sufficient margin to the surge limit is known. A method for operating a gas turbine, with which the margin to the surge limit at full-load operation is kept as constant as possible for the entire ambient operating range, is described in U.S. Pat. No. 6,226,974.
Furthermore, a method for operating a gas turbine is proposed in U.S. Pat. No. 6,794,766, in which during the quasi-steady-state operation at low aerodynamic speed the variable compressor guide vanes are closed in order to realize a sufficient surge limit margin. The surge limit margin in this case is selected large enough for the variable compressor guide vanes to be able to be opened in the case of an underfrequency event compared with the nominal position at mechanical design speed. As a result, the speed-dependent power output drop of the gas turbine can be at least partially compensated and the grid can be supported better.
A design and operation with large surge limit margin in the case of known gas turbines during normal operation, i.e. during operation at nominal speed in the ambient operating window, leads to a suboptimal compressor efficiency. Moreover, the power output potential of the gas turbine in wide operating ranges cannot be fully utilized.